6-(3-Dimethylaminopropionyl)forskolin has already been known as disclosed in EP 222,413-A and the like.
The present inventors attempted to make this compound into an injection. As a result, it was found that this compound is unstable in the state of aqueous solution and hence it must be made into a powdery preparation such as a lyophilized preparation, and that this compound has a fault that a lyophilized preparation prepared merely by dissolving this compound into water and lyophilizing it lowers its resolubility in the lapse of time during storage.